In the field of optical communication, multichannel configurations have been developed rapidly with the appearance of wavelength division multiplexing (WDM) communication systems. When channel-by-channel functional controls are attempted accordingly, for example, to uniformize the channel powers or to perform route switching, optical devices as many as correspond to the number of channels become necessary.
Under the circumstances, there has been a growing need in recent times for small-sized optical circuit components that are applicable to optical switches and the like and are capable of high density integration. Of such circuit components, planar lightwave circuit devices (PLCs) using a thermo-optical phase shifter have the advantages of having excellent manufacturability and integratability and being advantageous for functional sophistication and large scale integration since their manufacturing processes can use semiconductor circuit manufacturing technologies.
A thermo-optical phase shifter is typically manufactured in the following manner. Initially, an optical waveguide composed of a clad layer and a core is formed on a substrate. A metal thin film or the like is then deposited on the optical waveguide, and is processed into a thin wire shape along the optical waveguide so that the resulting wire-shaped metal thin film can be energized. When electric power is applied from an external power supply to the wire-shaped metal thin film, heat occurs from the electric resistance of the wire-shaped metal thin film. In other words, the wire-shaped metal thin film functions as a heater for heating the optical waveguide. The heat generated by the heater is transferred through the clad layer to reach the core of the optical waveguide. As a result, the optical waveguide increases in refractive index in the area heated by the heater, and the change in the refractive index can be used to construct a functional optical device such as an optical switch.
A single optical circuit may include a plurality of thermo-optical phase shifters for the sake of multichannel configuration. In such cases, the power consumption of the entire optical circuit can be extremely high if each single thermo-optical phase shifter consumes high power. Among thermo-optical phase shifters that have been put to practical use heretofore, a Mach-Zehnder optical switch, for example, consumes around 400 mW of power to switch a single channel of light. For example, to control a 40-channel optical communication circuit with an optical switch using the foregoing thermo-optical phase shifter on each channel, a maximum power of 40×400 mW=16000 mW=16 W is required. In fact, optical switches near as much as the square of the number of channels are often needed in order to perform route setting and the like on all the channels. In such a case, the power consumption can reach up to 40×40×400 mW=640 W.
For the purpose of preventing the dissipation of the heat generated by the heater to the substrate so as to reduce the power consumption of the thermo-optical phase shifter, it has been proposed to remove a sacrifice layer located under the optical waveguide to construct the optical waveguide in a bridge structure (for example, see JP-A-2004-37524 [PTL1]). Such a technology makes it possible to effectively confine the heat generated by the heater within the optical waveguide for use, thereby reducing the power consumption of the thermo-optical phase shifter significantly.
Now, high-Δ optical waveguide technology has been astonishingly advancing recently with the aim of achieving integrated optical devices of even smaller sizes and higher densities. That is, the confinement of light into the optical waveguide core is further intensified to minimize the bending loss so that curved waveguides, which have been redundant in conventional optical waveguide devices, can be reduced or shortened significantly. Conventional optical waveguide core materials have limitations in achieving such a high-Δ optical waveguide, and SiON, Si, and the like are used as new core materials.
As described above, the device performances strongly desired at present include low power consumption and small-sized integration, and the foregoing two technologies, namely, the heat insulation technology using a bridge structure and the high-Δ optical waveguide technology are attracting attention as important technologies.
The combination of such technologies has the following technical problems, however.
Optical waveguide core materials for constructing a high-Δ optical waveguide have coefficients of thermal expansion far different from those of clad materials. Manufacturing processes are also quite different from theretofore. Suppose, for example, that SiON, which has a wide range of adjustment in refractive index and is suitable for high refractive indexes, is used as the optical waveguide core material. In such a case, infrared absorptions due to O—H bond and N—H bond unique to SiON occur in the vicinity of 1500 nm, an infrared wavelength band for use in optical communication, with an increase in absorption loss. To reduce the absorption loss, annealing needs to be performed at temperatures above 1000° C. Such high temperature annealing makes the glass film closely packed, allowing the manufacturing of highly reliable devices. The high temperature annealing performed with different types of materials in contact with each other, however, can cause extremely large residual stress corresponding to a difference between the coefficients of thermal expansion inside the optical waveguide. Consequently, even if the same bridge structure is formed as in conventional low-Δ waveguides, the bridge formation simultaneously allows the optical waveguide portion that has been supported by the substrate to make a stress deformation freely, which results in the problem that the bridge structure of the optical waveguide breaks. The breakages are often in the connections between the bridge structure portion and a fixed portion that supports the bridge structure portion on the substrate at both ends. The reason of such breakages is that the force of the bridge structure to make a stress deformation concentrates on the connections. Moreover, as specifically described in an example in the foregoing PTL1, if the bridge structure portion including the optical waveguide and the heater has a straight shape, it is unpredictable whether the direction of the stress deformation is toward or away from the substrate, and whether or not the deformation occurs in a direction parallel with the substrate. The direction of the deformation varies depending on the surrounding circumstances at the moment when the bridge structure portion is separated from the substrate, and depending on the structural balance. Consequently, even if the bridge structure remains unbroken, it is difficult to manufacture with stability a thermo-optical phase shifter that has low power consumption performance and high optical characteristics.
PTL1 describes that the bridge structure portion is supported by a support column in order to reduce the degrees of freedom of the bridge structure portion in vertical directions (FIG. 3, etc).
PTL1 includes the description that the heater may have a curved shape if the heater is capable of generating heat to bring the optical waveguide core up to a desired temperature and induce a change in refractive index (paragraph [0095]). PTL1 also includes the description that the optical waveguide core is formed in a curved shape (claim 1, etc.). Nevertheless, PTL1 includes neither the description that the bridge structure portion is formed in a curved shape or, in particular, the description that the bridge structure portion using a high-Δ optical waveguide is formed in a curved shape, nor the description that the shape of the bridge structure portion is specified to reduce the residual stress and avoid a break of the structure, or to reduce the residual stress and avoid a break of the structure of the bridge structure portion using a high-Δ optical waveguide in particular.
Meanwhile, an optical waveguide for use in optical communications has also been proposed in which a curved portion is formed in a ridge structure so that the radius of curvature of the curved portion can be reduced even with suppression of light loss (for example, see JP-A-2004-287093 [PTL 2]). Nevertheless, PTL 2 includes neither the description that a bridge structure including an optical waveguide and a heater is formed, nor that the shape of the bridge structure is specified to reduce the residual stress and avoid a break of the structure, or to reduce the residual stress and avoid a break of the structure of the bridge structure portion using a high-Δ optical waveguide in particular.
PTL 1: JP-A-2004-37524
PTL 2: JP-A-2004-287093